1. Field of the Invention
The present invention relates to an illumination intensity correcting circuit for correcting the intensity of a video signal or three primary color signals.
2. Description of the Related Art
FIG. 1 is a schematic block diagram of a computer display apparatus of the related art. This computer display apparatus 100 includes a signal amplifier circuit 20, a cathode ray tube (hereinafter referred to as a CRT) 30, and a deflection circuit 40. The signal amplifier circuit 20 includes first to third amplifier circuits 21 to 23. The CRT 30 includes an electron gun 34. The first amplifier circuit 21 receives as an input a red primary color signal SR to amplify the same and supplies the resultant signal SR2 to a cathode 31 of the electron gun 34. The second amplifier circuit 22 receives as an input a green primary color signal SG to amplify the same and supplies the resultant signal SG2 to a cathode 32 of the electron gun 34. The third amplifier circuit 23 receives as an input a blue primary color signal SB to amplify the same and supplies the resultant signal SB2 to a cathode 33 of the electron gun 34.
The electron gun 34 generates electron beams BR, BG, and BB according to the output signals SR2, SG2, and SB2 correspondingly and emits the electron beams BR, BG, and BB to a fluorescent screen 35. This fluorescent screen 35 is made of a fluorescent material that is coated on the inside of the front glass of the CRT 30. The deflection circuit 40 receives as inputs horizontal and vertical synchronizing signals SH and SV and generates a magnetic force by a deflecting coil (yoke) in accordance with the signals SH and SV to change magnetic field force in the CRT 30 so that horizontal and vertical scanning by the electron beams from the electron gun 34 are carried out in the CRT 30. The three primary color signals SR, SG, and SB are analog output signals for example from digital-to-analog converters which are supplied with digital signals respectively from frame buffers used for the three primary color signals or from color look-up tables (CLUTs).
Known in the art is a xe2x80x9ccurve fitting circuitxe2x80x9d for compressing the amplitude of an input signal by approximating the signal characteristic by a series of breakpoints connected by straight lines. For example, the curve fitting circuit is formed by a parallel-connected circuit which consists of a reference resistor (first resistor) and a serial-connected circuit that includes a diode, a second resistor, and a constant-voltage source. This constant-voltage source generates a reference voltage. If the terminal voltage of the reference resistor is smaller than the reference voltage, the diode is supplied with a reverse voltage and turned OFF, whereby the parallel-connected circuit is given a first electric resistance the same as that of the reference resistor. If the terminal voltage of the reference resistor is larger than the reference voltage, the diode is supplied with a forward voltage and turned ON, whereby the parallel-connected circuit is given a second electric resistance of the reference resistor and the second resistor connected in parallel. If the parallel-connected circuit is used as a load resistor, the electric resistance of the load resistor is changed before and after the reference voltage, so the parallel-connected circuit can form a curve fitting circuit.
However this curve fitting circuit is structured so that the diode is supplied with a reverse voltage and forms a depletion layer capacitance when the reverse voltage is supplied. Since the resistance of the parallel-connected circuit differs for a low frequency component and a high frequency component of the same input signal due to the capacitance of the depletion layer, this curve fitting circuit is not well suited for high speed operation.
Moreover the diode resistance changes according to the applied voltage of the reference resistor due to the non-linear characteristic of the diode, so this curve fitting circuit may suffer from distortion near each breakpoint in the signal characteristic.
A television receiver is sometimes provided with an illumination intensity correcting circuit for adjusting the relationship between the three primary color signals or video signal and the brightness of the image. For example, use is sometimes made of a gamma correcting circuit. As the curve of the gamma characteristic, use may be made of a convex secondary degree curve. Because the gamma correcting circuit of the related art forms a secondary degree curve by using capacitor elements, it is difficult to perform gamma-correction on the high frequency component of a video signal or three primary color signals.
An object of the present invention is to provide an illumination intensity correcting circuit which can perform gamma-correction on the high frequency component of a video signal or three primary color signals.
Another object of the present invention is to provide an illumination intensity correcting circuit which can prevent distortion near the breakpoints in the signal characteristic.
To achieve the above objects, according to a first aspect of the present invention, there is provided an illumination intensity correcting circuit for performing gamma-correction on a video signal comprising parallel-connected differential amplifier circuits for generating signals corresponding to the difference between a voltage of the video signal and the reference voltages of the differential amplifier circuits and a load resistor with one terminal connected to an input terminal of a source voltage and with another terminal connected to the output terminals of the differential amplifier circuits, wherein a curve fitting circuit whose amplification factor is changed before and after each breakpoint voltage is formed by the differential amplifier circuits and the load resistor, the reference voltages of the differential amplifier circuits are set so that at least two breakpoint voltages are arranged in the range of the voltage of the video signal, and the amplification factors of the differential amplifier circuits are set so that the amplification factor of the curve fitting circuit in the range of the signal voltage between the two breakpoint voltages is smaller than the amplification factor of the curve fitting circuit outside the range between the two breakpoint voltages.
To achieve the above objects, according to a second aspect of the present invention, there is provided an illumination intensity correcting circuit for performing gamma-correction on a video signal comprising differential amplifier circuits for generating signals corresponding to the difference between a voltage of the video signal and the reference voltages of the differential amplifier circuits; a synthesizing circuit including a differential amplifier circuit to synthesize output signals of the differential amplifier circuits; and a load resistor with one terminal connected to an input terminal of a source voltage and with another terminal connected to an output terminal of the synthesizing circuit, wherein a curve fitting circuit whose amplification factor is changed before and after each breakpoint voltage is formed by the differential amplifier circuits, the synthesizing circuit, and the load resistor, the reference voltages of the differential amplifier circuits are set so that a breakpoint voltage is arranged in the range of the voltage of the video signal, and the amplification factors of the differential amplifier circuits are set so that an amplification factor of the curve fitting circuit in a range of the signal voltage lower than the breakpoint voltage is larger than the amplification factor of the curve fitting circuit in the range higher than the breakpoint voltage.
To achieve the above objects, according to a third aspect of the present invention, there is provided an illumination intensity correcting circuit for performing gamma-correction on three primary color signals comprising first, second, and third correcting circuits respectively receiving as inputs red, green, and blue primary color signals and comprising parallel-connected differential amplifier circuits for respectively generating signals corresponding to the difference between voltages of the three primary color signals and the reference voltages of the differential amplifier circuits and a load resistor with one terminal connected to an input terminal of a source voltage and with another terminal connected to the output terminals of the differential amplifier circuits, wherein a curve fitting circuit whose amplification factor is changed before and after each breakpoint voltage is formed by the differential amplifier circuits and the load resistor, the reference voltages of the differential amplifier circuits are set so that at least two breakpoint voltages are arranged in the range of the voltages of the three primary color signals, and the amplification factors of the differential amplifier circuits are set so that the amplification factor of the curve fitting circuit in the range of the signal voltage between the two breakpoint voltages is smaller than the amplification factor of the curve fitting circuit outside of the range of the two breakpoint voltages.
To achieve the above objects, according to a fourth aspect of the present invention, there is provided an illumination intensity correcting circuit for performing gamma-correction on three primary color signals comprising first, second, and third correcting circuits respectively receiving as inputs red, green, and blue primary color signals and comprising differential amplifier circuits for respectively generating signals corresponding to the difference between voltages of the three primary color signals and the reference voltages of the differential amplifier circuits, a synthesizing circuit including a differential amplifier circuit to synthesize output signals of the differential amplifier circuits, and a load resistor with one terminal connected to an input terminal of a source voltage and with another terminal connected to an output terminal of the synthesizing circuit, wherein a curve fitting circuit whose amplification factor is changed before and after each breakpoint voltage is formed by the differential amplifier circuits, the synthesizing circuit, and the load resistor, the reference voltages of the differential amplifier circuits are set so that a breakpoint voltage is arranged in the range of the voltages of the three primary color signals, and the amplification factors of the differential amplifier circuits are set so that an amplification factor of the curve fitting circuit in a range of the signal voltage lower than the breakpoint voltage is larger than the amplification factor of the curve fitting circuit in a range higher than the breakpoint voltage.
Explaining the invention from another perspective, the first and third aspects of the present invention form curve fitting circuits which include parallel-connected differential amplifier circuits and a load resistor with one terminal connected to an input terminal of a source voltage and with another terminal connected to the output terminals of the differential amplifier circuits. The differential amplifier circuits generate signals corresponding to the difference between the voltage of the input signal and the reference voltages of the differential amplifier circuits. The differential amplifier circuits are parallel-connected, and the output signals of the differential amplifier circuits are synthesized so that a curve-fitting input-output characteristic is formed. The first aspect of the present invention provides an illumination intensity correcting circuit for gamma-correction of a video signal. The third aspect of the present invention provides an illumination intensity correcting circuit for gamma-correction of three primary color signals.
The second and fourth present inventions form curve fitting circuits each of which includes differential amplifier circuits, a synthesizing circuit that synthesizes output signals of the differential amplifier circuits, and a load resistor with one terminal connected to an input terminal of a source voltage and with another terminal connected to an output terminal of the synthesizing circuit. The differential amplifier circuits generate signals corresponding to the difference between the voltage of the input signal and the reference voltages of the differential amplifier circuits. The output signals of the differential amplifier circuits are synthesized by the synthesizing circuit so that a curve-fitting input-output characteristic is formed. The first aspect of the present invention provides an illumination intensity correcting circuit for gamma-correction of a video signal. The third aspect of the present invention provides an illumination intensity correcting circuit for gamma-correction of three primary color signals.